It is well known that microwave intruder alarms and other microwave systems such as duplex communication links have been produced with a common antenna connected to both a transmitter and a receiver. For example, in microwave intruder alarms, it has been proposed that a microwave transmitter and receiver be connected to a common antenna such that the receiver senses doppler shifted returns from the transmitter radiation in the presence of a moving object. A disadvantage of such prior proposals is that they have required relatively complex microwave waveguide plumbing arrangements in order to achieve satisfactory performance with a common antenna, such complex arrangements leading necessarily to a physical structure which is complex and uncompact in nature.